The present invention relates to the use of conical crushers for the comminution of mineral material, and more specifically, to the use of a conical crusher in a grinding mode, i.e., to produce a higher percentage of fine sized product at a given throughput capacity.
In the comminution of mineral materials, the grinding step, or the reduction of the size of crushed particles to a relatively fine sized product, is commonly performed by tumbling rod or ball mills, and is conventionally accepted as one of the more, if not the most energy intensive step in the comminution process. As a result, efforts have been made to reduce energy consumption in the grinding operation.
One such suggested solution is embodied in U.S. Pat. No. 4,537,287 to Schoenert, who discloses performing grinding using a pair of parallel compression rollers oriented to have a relatively narrow gap therebetween, through which is inserted a flow of feed material. The rollers are designed to exert sufficient compressive forces on the material between the rollers to effect comminution of the feed material. In some cases, the compressive force of the rollers results in the creation of agglomerates or briquettes. The comminution system disclosed by Schoenert is inefficient in that it only utilizes a single step stressing process, which has been shown to consume higher energy for a given reduction ratio than a multi-step stressing process for the same given reduction ratio. Although devices such as Schoenert's, commonly known as roll presses, have been suggested for use in the cement industry for the comminution of "clinker" material, the conventional rod or ball mill still needs to be used as a finishing step in the production of fine materials after the roll press. Also, the roll press has not received commercial acceptance in the comminution of relatively harder materials such as taconite, copper, etc.
Conical crushers are normally used as secondary or tertiary stage comminution devices, and as such have not been used extensively for grinding. Commonly assigned U.S. Pat. No. 4,697,745 discloses that the setting of a conical crusher may be narrowed to increase the production of fines, and that the tightening or narrowing of the setting requires additional power to achieve equivalent crusher production rates. This additional power may be supplied by proportionately increasing the rotational speed of the eccentric. In addition, when the setting is narrowed beyond the design limits for a particular crusher unit, the designed crushing force in the lower margin of the bowl liner will be surpassed, causing the crusher to "bounce" through the generation of vibrations in the area of the adjustment ring. This crusher "bounce" has proved to be a significant obstacle to the use of conical crushers to produce high volumes of fine product.
Thus, there is a need for an energy efficient, stress managing method of operating a conical crusher to produce a significant volume of fines, and to essentially perform the grinding portion of a comminution circuit to enable the replacement of conventional ball mill or roll press grinding equipment.